Ova from fresh ovulations in mated and unmated hamsters will be homogenized and assayed for estrogen content. To correlate endocrinology with morphology, blood will be drawn for radioimmunoassays of estradiol, estrone and progesterone on days 3 and 4 of the hamster cycle (postestrous discharge equals day 1), the first seven days of pregnancy (covering implantation) and subsequently, the time of vascularization of the chorioallantoic placenta (day 9), the development of the definitive placenta, and prior to, during and after parturition. Each animal used will be autopsied immediately for assessment of physiological state, number and condition of ova etc. Hamster blastocysts, blastocysts and endometrium, as well as endometrium alone will be examined for steroidogenic activity; and a comparable study carried out in the rat or mouse. Antiserum to estradiol-17 beta and the antiestrogen CI-628 will be tested for individual effects on hamster implantation.